Hepatitis B core antigen (HBcAg) is a particle from the core of the hepatitis B virion (HBV), sometimes referred to as the Dane particle. HBcAg is a potent immunogen which typically elicits a strong immune response persisting for years after HBV infection has cleared and is found in serum of patients with chronic HBV infections.
The detection of antibody to hepatitis B core antigen (anti-HBc) is used to monitor the progress of HBV infections. Anti-HBc is found in serum shortly after the appearance of hepatitis B surface antigen (HBsAg) and, in acute hepatitis B, will persist after the disappearance of HBsAg and before the appearance of detectable antibody to HBsAg (anti-HBs). Therefore, in the absence of HBsAg and anti-HBs, anti-HBc may be the only serological marker of recent hepatitis B infection and potentially infectious blood.
About ten years ago, a large study of transfusion-related viral infections revealed that donors who were positive for anti-HBc were often implicated in the transmission of other infectious agents to recipients. Vyas and Perkins, New England J Med., 306: 749-750, 1981. Most notable was the finding that approximately 40% of the recipients of donor blood who subsequently developed non-A, non-B hepatitis infections (NANB) had received at least one unit of anti-HBc positive blood. While the scientific relationship of this association of NANB and anti-HBc has not been established, many scientists believe that it is an epidemiological association and that donors who have been exposed to hepatitis B are among those most likely to have been exposed as well to NANB. Because of the failure to date to identify a specific marker for NANB and because of the significant frequency of NANB in blood recipients, blood banks have turned to surrogate testing to improve the safety of the blood supply in the U.S. Detection of anti-HBc has been selected as one such surrogate test for lowering the transmission of NANB.
Immunoassays for the detection of anti-HBc are well known. Examples of such assays include the CORAB.RTM. radioimmunoassay and the CORZYME.RTM. enzyme immunoassay, both commercially available from Abbott Laboratories, North Chicago, Ill. These assays are competitive immunoassays in which anti-HBc in a biological sample competes with a constant amount of anti-HBc which is labeled with a detectable label for a limited number of binding sites on a solid phase coated with HBcAg. The proportion of bound labeled anti-HBc is inversely proportional to the concentration of anti-HBc in the biological sample.
The anti-HBc assays described above can be performed by one- or two-step methods. In the one-step procedure, a sample and labeled anti-HBc are mixed together and allowed to compete for HBcAg attached to a solid phase. In the two-step procedure the HBcAg on the solid phase is first reacted with the test sample and then with labeled anti-HBc in a second step. In either method, the amount of labeled anti-HBc bound to the solid phase is measured, and a reduction in the amount of labeled antibody is indicative of an antibody positive sample which is able to block binding of the labeled antibody.
False positives represent a problem in screening donor blood for anti-HBc because there are a large number of specimens which are positive only for anti-HBc with no other hepatitis marker present and no history or symptoms of disease. There is no simple way to confirm that these specimens are true positives. This is particularly troublesome when anti-HBc testing is used as a surrogate screening for NANB because the epidemiologic connection between the marker and the disease is not strong (85% of recipients of anti-HBc positive blood do not develop a known infection). Therefore, it is important to discriminate between anti-HBc positive specimens that relate to NANB and those which do not or are false positives.